Shell and tube heat exchangers have an array of tubes extending between and through two spaced apart tube sheets surrounded by a shell. The shell is provided with an inlet and an outlet so that a suitable heat exchange liquid or gas can be circulated through the shell to cool or heat a liquid flowing through each tube.
Each end of the array of tubes can be left open, or exposed, for use in some processing operations. For other operations, one or both ends can be enclosed by a liquid retaining header, which may or may not have a removable cover or access port.
Although shell and tube heat exchangers are generally used to heat a liquid feed stream, they can be used for cooling such a stream. Shell and tube heat exchangers of the described types can be used as freeze exchangers for producing fresh water from brackish water and seawater, for concentrating fruit and vegetable juices, and industrial crystallization processes. As the liquid flows through each tube, it can be cooled enough to crystallize a solid from the liquid. Thus, by cooling seawater, ice is obtained which when separated, washed and melted provides potable water. When a fruit or vegetable juice is similarly chilled, ice forms and is removed to provide a concentrated juice.
Freeze exchangers of the described type can use any cooling fluid on the shell side to cool a liquid flowing downwardly through the tubes. The fluid can be fed through one end and removed through the other end of the freeze exchanger in a substantially unidirectional flow. Some suitable cooling fluids are refrigerant gases such as ammonia and Freon brand refrigerants.
When a refrigerant is used, it is generally liquefied first and then fed as a liquid to the shell side of the freeze exchanger through an inlet port in the lower part of the shell. A volume of refrigerant liquid is maintained on the shell side with a refrigerant vapor volume above the liquid level. The refrigerant liquid level is usually maintained about at the lower part of an outlet port in the shell below the upper tube sheet. Accordingly, essentially refrigerant vapor and little liquid is withdrawn from the outlet port.
Actual operating experience has shown that the area of the tubes adjoining the refrigerant liquid-vapor shell side interface is exceptionally cold, and colder than other surface areas along the tube length. As a result, when an aqueous liquid flows downwardly through each tube and is cooled, there is a strong propensity for ice to deposit and build up as a ring on the inside of the tube. The ice ring restricts flow through the tubes and sometimes totally blocks the tubes. The capacity of the freeze exchanger is drastically reduced when the tubes are restricted or blocked. To obtain normal capacity, the freeze exchanger must be taken out of service and the ice melted before it is replaced in service. This represents an operating loss. There is a need, accordingly, for a freeze exchanger which avoids or reduces the formation of ice in freeze exchanger tubes, especially ice deposits favored by having a shell side refrigerant liquid-vapor interface in direct contact with the tubes.